Spring mattress



FIG. 1 is a perspective view of a spring mattress showing my new design;

FIG. 2 is a front view of the spring mattress shown in FIG. 1;

FIG. 3 is a rear view of the spring mattress shown in FIG. 1;

FIG. 4 is a left side view of the spring mattress shown in FIG. 1;

FIG. 5 is a right side view of the spring mattress shown in FIG. 1; and,

FIG. 6 is a top plan view of the spring mattress shown in FIG. 1.

The bottom view is omitted as forming no part of the claimed design. 

CLAIM The ornamental design of a spring mattress, as shown and described. 